This invention relates to an improvement in the electrochemical separation of uranium and plutonium from spent metal-clad fuel pins, in which essentially complete recovery of uranium and plutonium is achieved at high reaction rates and acceptable levels of anode efficiency, without the need for good electrical contact between the fuel pins and the power source at the anode, and with effective separation of non-uranic fission byproducts, as well as cladding materials, so that high purity uranium and plutonium are recovered at the cathode, while preserving the operating metal parts of the electrochemical cell from corrosion.
The recovery of fissionable materials such as uranium and plutonium from spent reactor fuel is often carried out using electrochemical cells of the types described in U.S. Pat. Nos. 4,596,647 and 2,951,793, as well as U.S. Ser. No. 07/117,880 (filed Nov. 5, 1987). U.S. Pat. No. 4,596,647, for example, describes an electrochemical cell incorporating many features that have been found useful. In the process for which that cell is intended, spent metal-clad fuel pins chopped into disc-shaped pieces are loaded into a perforated metal basket which forms the anode of the cell. The basket is immersed in a pool of electrolyte such as an eutectic salt of CaCl.sub.2 -BaCl.sub.2 -LiCl, also containing U.sup.+3 and U.sup.+4 cations and plutonium cations in solution, which pool in turn floats upon a lower pool of molten cadmium. Both the perforated basket containing the fuel pins and the pool of cadmium are electrically connected as anodes. The cathode assemblies, of which one or more may be employed, may consist of a metallic rod (for uranium deposition), typically of molybdenum/-tungsten or mild steel, contained within a perforated, non-conductive cylindrical casing and provided with means for rotating the rod and casing assembly so as to agitate the electrolyte pool, as described in U.S Pat. No. 4,596,647. Alternatively, one or more molten cadmium cathodes of the type described in U.S. Ser. No. 07/117,880 may be used to recover substantially pure uranium followed by a mixture of uranium and plutonium in a two-step operation.
In operation, the anode basket is first lowered into the electrolyte pool and an electrical potential is imposed between the anodes and the cathode, resulting in the electrochemical oxidation of spent uranium and plutonium from the anode and reduction of their cations to metallic uranium and plutonium at the cathode. In this manner, the uranium and plutonium can be dissolved from the pins without using voltages high enough to cause corrosion of the anode basket material and other fittings, or attack on the metal cladding of the pins. But a significant fraction of the power applied to the anode is used to produce U.sup.+4 cations instead of U.sup.+3 cations, a condition that worsens as the voltage is increased. Production of U.sup.+4 cations that eventually reach the cathode in that higher oxidation state lowers the anodic efficiency (the ratio of uranium and plutonium actually dissolved to the theoretical amount that would have dissolved if all of the current used had actually oxidized fissionable material) below the level that could be achieved if only U.sup.+3 cations were produced and transported to the cathode, because more amp-hours of current are required to oxidize one mole of uranium to U.sup.+4 than to U.sup.+3. Also, when U.sup.+3 is oxidized to U.sup.+4 at the anode, dissolution of fuel does not take place, and current is consumed.
It is an object of the present invention to increase the rate of anodic dissolution in the anode basket of such an electrochemical cell. Another object is to provide for complete removal of the uranium and plutonium (including uranium and plutonium that have reacted with the metal cladding of the pins) without need to immerse the anode basket in the cadmium pool. (In prior art devices, even immersing the anode basket in the cadmium pool left undissolved an insoluble layer of uranium and plutonium that had reacted with the cladding.) It is a further object of this invention to improve the anodic efficiency of the operation by reducing the transport of U.sup.+4 cations to the cathode, and/or once having formed U.sup.30 4 at the anode, to cause the U.sup.+4 to react with elemental U in the spent fuel, producing further fuel dissolution. Yet another object of this invention is to provide an improved anode basket design that operates satisfactorily notwithstanding poor electrical contact between the basket and the spent fuel pins. A final object of this invention is to provide an anode basket design that can be easily and completely cleared of electrolyte salt solution after the dissolution process has been completed, thus facilitating recovery of clean metal cladding, as a process waste.